1. Field of the Invention
The present invention generally relates to mobile telecommunications and, more particularly, to air-to-ground telecommunications through a sequence of ground stations through which maximum quality communications may be maintained during the course of a flight.
2. Description of the Prior Art
Recent progress in high-frequency radio communications has enabled the support of many simultaneous communication links such as are involved in telecommunications. In particular, mobile telephony has effectively replaced citizens band radio as the communication link of choice when one or a plurality of communicating parties are located in vehicles such as automobiles, boats or airplanes. In fact, the limited number of channels available limited the privacy of such communications and use of such communications was therefore inappropriate to many of the types of communication commonly conducted by telephone. More recently, however, so-called cellular telephony systems, characterized by the overlapping reception fields have made the privacy and convenience expected of stationary communications utilities available to the mobile communications market.
In cellular telephony systems, the size of the reception area of a single cell is very large relative to the speed of vehicle travel. That is, due to the overlapping arrangement of the reception fields, a channel will normally be available through at least one cell for a given location of the mobile transmitter. Further, while a call may be extended, it is relatively unlikely that a call will continue while the vehicle moves beyond the range of the cell in which it is begun or even so close to a fringe area of the cell that service will be significantly impaired.
The same is not true of air-to-ground telecommunications due to the increased speed of the aircraft with which the communication may be held. While air-to-ground telecommunications are in many ways similar to cellular telephony systems, the speed of the mobile vehicle and the average duration of the call (since the caller is seldom the operator of the vehicle as is often the case in communications to automobiles) are significant in comparison to the diameter of the zone over which quality communication may be maintained. In addition, atmospheric conditions may be more likely to increase packet error rate and reduce received signal power. Doppler shift effects which may reduce transmission quality may also be significant in air-to-ground telephony but of negligible effect at the normal speeds of ground or water based vehicles.
The concept of altering communication links while a particular call is in process is complex but not unknown in telephony. However, such transfers are often accomplished in telecommunication utilities for the benefit of the utility in accommodating traffic levels at any particular time. Ideally, the quality of service provided by stationary facilities should not change but, in practice, quality may be affected by atmospheric conditions where microwave or satellite links are employed. Even where communication links must be changed during the course of a call, the time delay inherent in each communication link is constant in stationary systems to a relatively high degree of accuracy regardless of atmospheric conditions. Wired links will be largely immune from variation in propagation delays due to variation of atmospheric conditions. Therefore, the transfer of communications links can usually be accomplished fairly simply and largely without the users becoming aware of the transfer, even when complex hand-off procedures are employed. Under normal circumstances in a stationary system, a second communication link is established in a synchronized fashion and time correlated or piggy-backed with the first. Then, at an appropriate time, such as when the packet rate is low, the hand-off is accomplished by substituting the second link for the first. The slight time shift is not noticeable to the users since it will usually be manifested in the slight shortening or lengthening of a period of relative silence, for which the respective users have no reference.
In a cellular system, however, to avoid the loss of a call when a mobile cellular unit travels beyond the range at which a base station can maintain a good quality communication link, a base station controlled xe2x80x9chand-off mechanism has been developed. In this hand-off mechanism, a channel unit of the base station measures the quality of the signal from the mobile unit and notifies the base station controller when quality drops below a predetermined level. The base station controller will contain a list of adjacent base stations and, when the received signal quality becomes low, commands these adjacent base stations to scan for the radio link on which the call is being conducted. It should be noted that cellular systems are synchronously operated with a national clock and the scanning process requires testing received signals for signal from the mobile unit in the same signal transmission frame at a plurality of adjacent stations. When such a signal is found at another base station, the received signal quality is tested and the hand-off is controlled by one of the base stations.
So-called mobile assisted hand-off systems are also known which are similar to the hand-off mechanism for cellular telephony. In mobile assisted hand-off systems, the mobile unit is directed to scan control frequencies of the adjacent base stations and to notify the current base station of the strongest candidates. The base station then directs only those candidate base stations to scan for the signal of the mobile station.
Both of these systems require many complex operations and numerous communications between base stations as well as requiring the ground-based system to be operated in a synchronized manner. However, this complexity is tolerable in view of the low likelihood that a hand-off will be required.
In contrast, with mobile communications involving a high speed aircraft the time during which an aircraft remains within good quality communication range of a base station is very much reduced and the likelihood that a hand-off will be required during a particular call is very much increased. Further, time relationships and propagation delays are constantly changing. Therefore, it is difficult to provide a change of communications links which is xe2x80x9cseamlessxe2x80x9d and not readily detectable by an operator since such a change is often accompanied by momentary high noise levels or the omission or repetition of a portion of the communication or a very audibly noticeable change in the quality of the communication link. The changing time relationships also would complicate the scanning for the call from the mobile unit if a complex, synchronous system such as are used in cellular or mobile assisted hand-off systems were to be employed.
It is therefore an object of the present invention to provide a technique of alteration of communication links in which the quality of communication links will be closely matched.
It is another object of the present invention to provide a system for substitution of communication links in which the communication will be substantially xe2x80x9cseamlessxe2x80x9d and without noticeable time error.
In order to accomplish these and other objects of the invention, a method is provided for substitution of communication links in a seamless manner including the steps of establishing a first communication channel carrying a call over a first communication link, establishing a second communication channel over a different communication link, establishing a call over the second communication channel corresponding to the call carried by said first communication channel, comparing quality of transmission in said first and second channels, and transferring voice signals of the call from the first communication channel to the second communication channel in response to the comparing step.
In accordance with another aspect of the invention, a method is provided for substitution of communication links between a mobile telephony unit and ones of a plurality of ground stations in a seamless manner including the steps of comparing quality of reception at said mobile unit over at least first and second communication links between the mobile unit and respective first and second ground stations, and transferring call signals between the first and second communication links in response to said comparing step.